Why not Las Vegas? The city is an extraodinary city, clearly has the hotel space, and is strategically located in a Republican leaning state that could be competitive. The casinos should not be a reason to say no.

Seattle did not express interest in hosting the convention. You would have to ask city officials why that was. (My guess is they were concerned about cost, and maybe did not have enough close-in hotel rooms - these are usual concerns for many cities).